For installation of optical fibers, a method of binding or twisting several optical fibers into a cable, and then installing this cable has been mainly used. In this cable installation method, optical fibers much more than required at the point of installation are generally installed in advance with expectation of future demands.
However, since more various kinds of optical fibers are required according to the trend of new communication environments and there have been developed high performance communication systems suitably coping with communication capacity even in restricted optical fiber installation environments, it cannot be considered desirable that a large amount of optical fibers are installed in advance just with expectation of future demands. In particular, in aspect of a user terminal, namely an access network or a premise wiring, a mode of an optical fiber or cable in future cannot be decided at the present point of time. Thus, if a large amount of optical fibers are installed in advance with incurring much expense, there may be a waste of money if a mode of an optical fiber or cable is changed in future.
In order to solve the above problems, a method for installing an optical fiber unit having several optical fiber strands collected therein by air pressure is widely used. This air blown installation method was firstly proposed by British Telecom Co. (see U.S. Pat. No. 4,691,896) in 1980. In this air blown installation method, a polymer installation tube, called a micro tube or duct, having specific constitution and sectional shape is installed at an optical fiber installation spot in advance, and then an air blown optical fiber unit (hereinafter, referred to just as ‘an optical fiber unit’) is inserted into the micro tube or duct as much as required by air pressure. If optical fibers are installed using the above optical fiber installation method, many advantages are ensured, namely easy installation and removal of optical fibers, reduced costs for initial installation, and easy improvement of performance in future.
FIG. 1 is a schematic view showing an optical fiber unit installation device used in the above air blown installation method. Referring to FIG. 1, the installation device successively inserts an optical fiber unit 1 from an optical fiber unit supplier 2 into an installation tube 4 connected to an outlet C of a blowing head 5 by using a driving roller 3 and a pressing means 6, and at the same time blows compressed air toward the outlet C of the blowing head 5 by using the pressing means 6. Then, the compressed air flows at a fast rate toward the outlet C, and accordingly the optical fiber unit 1 introduced into the blowing head 5 is installed in the installation tube 4 by means of a fluid drag force of the compressed air.
In order to ensure desirable installation of the optical fiber unit 1 in the air blown installation method, the fluid drag force of the compressed air should be great.
The fluid drag force F may be expressed as follows.
                                          F            →                    drag                =                              PR            1                    ⁢                      R            2                    ⁢                                    ⅆ              P                                      ⅆ              L                                                          Equation        ⁢                                  ⁢        1            
(P: compressed air pressure, R1: inner diameter of the installation tube, R2: outer diameter of the optical fiber unit, L: length of the installation tube)
In the Equation 1, the inner diameter R1 of the installation tube and the outer diameter R2 of the optical fiber unit are already defined in standards. Thus, in order to maximize the fluid drag force F, it is preferred to form irregularity on the surface of the optical fiber unit for increasing a contact area between the compressed air and the optical fiber unit.
As a scheme for increasing a contact area between the compressed air and the optical fiber unit, various structures of optical fiber units having irregularity on their surfaces are disclosed in U.S. Pat. No. 5,042,907, U.S. Pat. No. 5,555,335, U.S. Pat. No. 5,441,813, U.S. Pat. No. 6,341,188 and so on.
Specifically, U.S. Pat. No. 5,042,907 and U.S. Pat. No. 5,555,335 disclose an optical fiber unit having glass beads attached on its outer surface. In addition, differently from the method using beads, U.S. Pat. No. 5,441,813 and U.S. Pat. No. 6,341,188 disclose an optical fiber unit having concave dimples formed on its surface by using foaming polymer materials and an optical fiber unit having irregularity on its surface by winding a fiber made of special materials.
However, the above conventional techniques do not have any disclosure in relation to height of the irregularity, thickness of the outer layer on which the irregularity is formed, and Young's modulus of the outer layer of the optical fiber unit, which give serious influences on the air blown installation characteristics. Thus, there still exist difficulties in fabricating an optical fiber unit with excellent installation characteristics in the prior art.